Surface infiltration of Dysprosium and Terbium is a new technology for surface layer treatment after a high performance NdFeB rare earth permanent magnet is sintered into a blank. Through infiltration of Dysprosium and Terbium into a depth range of 2-3 millimeters from the surface layer of the magnet, the thermal resistance and coercive force of the magnet can be prominently improved. Therefore, such an infiltration technology has been preliminarily applied in the field of manufacturing a high performance NdFeB rare earth permanent magnet.
Dysprosium fluoride or Terbium fluoride must be uniformly coated on the surface of the magnet before the surface infiltration of Dysprosium and Terbium. Thickness and uniformity of a coating layer affect the quality of subsequent Dysprosium and Terbium infiltration. If the coating layer is too thin, the infiltration layer will be not deep enough after the infiltration of Dysprosium and Terbium; if the coating layer is too thick, it will be a waste of material; and if the thickness of the coating layer is not uniform, each region of the magnet will have different behaviors after the infiltration.
When infiltration is performed on a magnet in vacuum at high temperature, a good adhesive force between a coating material and the magnet is needed. At present, an immersion method is a commonly used method for coating the magnet. For example, CN101845637A discloses a processing technology of modifying the sintered NdFeB magnet alloy, comprising the steps of: dissolving powder of a heavy rare-earth oxide or fluoride with an appropriate weight into an acid solvent with a proper concentration; taking out and baking the magnet after the magnet is immersed in the acid solvent for an appropriate time, then a magnet surface being covered with a thin layer of heavy rare-earth powder; and placing the magnet in an argon furnace first, performing an heat diffusion treatment, and then performing an annealing treatment. As another example, CN102181820A discloses a method for improving coercive force of NdFeB magnet material, comprising the steps of: a. firstly, preparing a mixture of rare-earth fluoride powder and anhydrous ethanol; b. coating the surface of the NdFeB magnet material with said mixture through immersion; c. placing the NdFeB magnet material of which the surface is coated with said mixture in a vacuum heating furnace and performing an infiltration treatment; d. at last, performing a tempering treatment. As another example, CN104388952A discloses a method for accelerating diffusion and infiltration of a Dy/Tb adhesive layer on the surface of a sintered NdFeB magnet, comprising the steps of: firstly, cleaning the surface of the NdFeB magnet, and adhering a Dy/Tb element on the surface of the cleaned NdFeB magnet through immersion; then performing heat treatment under a high pressure to make a rapid infiltration of the Dy/Tb element in a grain boundary of the magnet; and then performing heat treatment at a middle temperature so that boundary structures of the magnet are further improved, and ultimately obtaining the NdFeB magnet with high coercive force.
At present, an electroplating method is another commonly used method for coating the magnet. For example, CN103839670A discloses a magnet preparation method for improving coercive force of a sintered NdFeB permanent magnet, comprising the steps of: a) using a vacuum fast-setting technology to prepare a blank of an NdFeB alloy magnetic material; b) performing the following treatments on the blank in sequence: chamfering-rinsing-washing-surface modification-washing; c) electroplating the treated permanent magnet with a nickel/heavy rare earth composite coating; and d) placing the sintered NdFeB magnetic material in a vacuum heat treatment furnace for heat treatment.
At present, a manual atomizing spraying method is another commonly used method for coating the magnet. For example, CN104134528A discloses a method for improving magnetic properties of a sintered NdFeB flake magnet, comprising the steps of: spraying a suspension solution having heavy rare-earth elements and having viscosity of 0.1 to 500 mPa·s under normal temperature and pressure on a surface of the sintered NdFeB flaky magnet, and then baking the sintered NdFeB flaky magnet so that a coating layer with heavy rare-earth elements is obtained on the surface of the sintered NdFeB flaky magnet; and performing diffusion treatment and aging treatment on the baked NdFeB flaky magnet under inert gas atmosphere.
The above-mentioned methods have following disadvantages: (1) the thickness of the coating layer is not uniform; (2) working efficiency is low; (3) the powder of Dysprosium fluoride and Terbium fluoride is wasted seriously. Therefore, there is an urgent need for a spraying device which can meet the requirements of coating uniformly and saving spraying raw material.